Rendered object too grainy (Beginner)
It is because you have too few samples. Samples are the number of times any given pixel is rendered. By increasing the number of samples in the render settings tab, you can decrease the amount of grainyness, or 'noise' in the image. Increasing the number of samples also raises render times too. 

Also, if it's taking too long when rendering an image with very many samples, consider using less samples but with the denoiser.

To learn about the denoiser: 

https://docs.blender.org/manual/en/dev/render/cycles/settings/scene/render_layers/denoising.html

And

Proper Denoising?

Hope this helps!